In the communication industry, billions of feet of insulated metallic conductors are manufactured each year. These conductors have many users. For example, they are used to connect distributing frames inside central offices, as inside wiring to provide service to customer station apparatus, and are twisted into pairs and stranded into cables.
Insulation which covers the metallic conductors must be relatively free of defects if it is to prevent shorting between adjacent conductors and ground when the conductors are placed into service. Accordingly, in the manufacture of insulated conductors, it is customary to test the insulation for defects. Defects may include uninsulated portions of the conductor as well as splits in the insulation or inadequate thickness thereof. Depending on the kinds of defects and the number of each in a given length, the insulated conductor may be accepted or rejected.
Defects are classified as pinholes, faults, or bare wires. A pinhole is defined as a defect in the insulation which is not visible to the naked eye. An insulated conductor may include a plurality of these and still be acceptable for use. A second kind of defect if referred to as a fault, and a third is that referred to as bare wire. Typically, a pinhole has a length which is less than 0.03 inch; a fault, a length of at least 0.03 inch but no greater than 0.25 inch; and a bare wire, a length greater than 0.25 inch.
One of the common tests for defects is to apply a voltage across the insulation. The voltage is many times higher than that which will be encountered during the service life of the insulated conductor. Any insulation defect, which may be troublesome during service, is detected by the flow of an electrical current through the insulation at the defect.
To subject conductor insulation to a relatively high voltage test, apparatus which is referred to as a spark-tester is commonly employed. The spark-tester apparatus is positioned between a supply of the insulated conductor and a takeup and in the path of travel of the conductor. One end of the insulated conductor is connected to ground, and a relatively high voltage is impressed across the insulation of the conductor as it advanced from its supply to the takeup. Any defect in the insulation will be manifested by a spark or arc between the spark-tester apparatus and the metallic conductor at the point in the insulation where the defect occurs.
In one prior art spark-tester apparatus, a plurality of beaded-chain electrodes are connected to a high voltage source and are disposed in a path of travel of an insulated conductor. As the insulated conductor is moved past the chains, a high voltage potential is impressed across the insulation and causes an arc over between the chains and an uninsulated portion of the conductor 21. Such an apparatus is described in U.S. Pat. No. 2,753,521. While apparatus such as this has been used for a number of years, it does not allow the classification of defects which is required for a suitable disposition of the insulated conductor.
Another commercially available test set, which includes two separate power supplies, distinguishes only between a fault and a bare wire. One portion of the test set detects fault-to-ground current while the other includes two probes which are spaced about one inch apart. The probes cannot be placed closer than one inch without an inadvertent breakdown from one probe to another. A fault as defined by this test set is any defect which is less than one inch in length whereas a bare wire is classified as one having a defect length of one inch or more.
This kind of arrangement, which has been used for some time, has certain disadvantages. For example, the above-described one inch length separation point between categories of defects, on which is based the acceptance or rejection of insulated conductors, is not altogether satisfactory. Conductors having faults of 0.25 inch or longer have been known to short out, and yet in the factory, such an insulated conductor would not have been rejected because the defects were not greater than one inch in length. It appears that, those in this art have not been able to distinguish among defects in a manner which is consistent with performance requirements.
As should be evident, there is a need in the art for methods and apparatus for not only detecting in conductor insulation but also for being able to measure these defects to permit a meaningful classification. It is also important that a conductor having an excessive number of defects will be identified and either rejected or repaired. Moreover, such an apparatus should be relatively economical, and hopefully be able to be used to detect defects in cable jackets as well as in conductor insulation. Seemingly, these needs have not been met by the prior art.